1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for recording information on an optical information recording medium, reproducing the information recorded on the medium and/or erasing the information recorded on the medium. Such an information recording and reproducing apparatus is suitably used as an information recording and reproducing apparatus which uses a card-like information recording medium on which a plurality of linear information tracks are arranged in parallel.
2. Related Background Art
As a medium for recording information by using a light and reading the information thus recorded, disk-shaped, card-shaped and tape-shaped media have been known. Of those, the card-shaped optical information recording medium (hereinafter referred to as an optical card) is compact and light in weight, convenient to carry and has a large memory capacity. Accordingly, a big demand is expected.
FIG. 1 shows a plan view of such an optical card 101. Numeral 102 denotes an information record area, numeral 103 denotes an information track, numerals 104 and 104' denote track select area, and numeral 105 denote a home position of a light beam spot.
On the optical card, information is recorded as a line of optically detectable record bits (information track) by scanning the card by a light beam which is modulated by recording information and focused into a small spot. In order to exactly record information without trouble such as crossing of information tracks, it is necessary to control (autotracking or AT) an irradiation position of the light beam spot on the optical card in a direction normal to a scan direction. In order to irradiate the light beam as a stable small spot irrespective of curvature of the optical card or mechanical tolerance, it is necessary to control (auto-focusing or AF) the light beam spot in a direction normal to the optical card surface. Further, the AT and AF are required in the reproduce mode.
FIG. 2 shows a configuration of an apparatus for recording and reproducing information to and from the optical card. Numeral 106 denotes a motor for driving the optical card 101 in a direction of arrow, numeral 107 denotes a light source such as a semiconductor laser, numeral 108 denotes a collimeter lens, numeral 109 denotes a beam splitter, numeral 110 denotes an objective lens, numeral 111 denotes a tracking coil, numeral 112 denotes a focusing coil, numerals 113 and 114 denote condenser lenses, numerals 115 and 116 denote photo-electric conversion elements, numeral 117 denotes a tracking control circuit and numeral 118 denotes a focusing control circuit. Currents are supplied to the tracking coil 111 and focusing coil 112 by commands from the control circuits 117 and 118 in accordance with tracking signal and focusing signal detected by the photo-electric conversion elements 115 and 116 so that the objective lens 110 is driven to effect the AT and AF.
A method for recording and reproducing information is explained with reference to FIGS. 1 and 2. The light beam spot is initially at the home position 105. The light beam spot them moves on the track select area 104 in a direction u to find a record or reproduce track N, when the AT and AF are effected and the N is scanned in a direction r to record or reproduce information. When the light beam spot comes into the track select area 104', a large current is momentarily supplied to the tracking coil 111 (FIG. 2) so that the light beam spot is kicked to the track (N+1). Then, the track (N+1) is scanned in the reverse direction l to record or reproduce information. Depending on amount of information, the scan of the information track 103 by the light beam spot and the kick of the light beam spot in the track select areas 104 and 104' are repeated several times.
In such an information recording and reproducing apparatus, when the optical card 101 is reciprocally driven by the motor 106, vibration is generated in the directions of AT and AF, because the light beam spot may be off-tracked in the direction of AT from the information track due to skew of the information track relative to the contour of the optical card and a backlash of the optical card drive mechanism and the AT control attempts to compensate for such off-track. On the other hand, in the direction of AF, the light beam spot may be defocused from the record plane of the optical card due to curvature of the optical card and the backlash of the optical card drive mechanism, and the AF control attempts to compensate it.
It has been known that the amplitude of such vibration depends on a frequency. FIG. 3 illustrates such frequency dependency. It shows the dependency in the AT direction. It is assumed that the skew is .+-.100 .mu.m. Assuming that the vibration in the AT direction is generated merely by the skew, the frequency dependency of the amplitude of the vibration in the AT direction is represented by a, at a frequency up to a reciprocation frequency fs when the light beam spot scans, the amplitude is flat at 100 .mu.m, and above the frequency fs, the amplitude decreases at a rate of -12 dB/oct. In order to keep the deviation in the AT within .+-.0.1 .mu.m, an open loop gain G.sub.T of the AT servo at the frequency below the scan frequency ts is EQU G.sub.T =20 log (100/0.1)=60 dB
as shown by .alpha. in FIG. 3, and it decreases at the rate of -12 dB/oct at the frequency above the scan frequency.
However, the vibration in the AT direction is caused not only by the skew but also by the reversal of the reciprocal movement of the optical card. Such vibration occurs at a resonant frequency fp (fs&lt;fp). This is due to mechanical vibration of the drive mechanism by abrupt deceleration and abrupt opposite acceleration upon the reversal of the reciprocal movement of the optical card. The vibration in the AT direction including the vibration at the reversal is represented by b in FIG. 3. In order to keep the deviation of AT within .+-.0.1 .mu.m under the vibration at the reversal, the AT gain is raised as shown by .beta. in FIG. 3.
As a result, the AT is sufficiently attained at the reversal. However, at a time other than the reversal, the AT gain is higher than required, particularly in a high frequency band. Accordingly, the AT servo system is sensitive to a fine defect or dust on the surface of the optical card. This causes degradation of recorded or reproduced signal.
The same is true for the AF direction.
In the information recording and reproducing apparatus described above, the scan speed of the light beam spot in the .gamma. direction and l direction differs between the record mode and the reproduce mode. In the record mode, a relatively low scan speed is selected by the limitation such as record sensitivity of the record medium. Thus, the speed V.sub.W in the record mode is lower than a speed V.sub.R in the reproduce mode (V.sub.W &lt;V.sub.R). Since the scan distance in the record mode is equal to the scan distance in the reproduce mode, a frequency f.sub.w of the reciprocation in the scan in the record mode is lower than a scan frequency f.sub.R in the reproduce mode (f.sub.W &lt;f.sub.R).
On the other hand, in the information recording and reproducing apparatus described above, when the optical card 101 is reciprocally driven by the motor 106, vibrations are generated in the AT and AF directions. It has been known that the amplitude of the vibration depends on the frequency of the vibration. FIG. 4 illustrates such frequency dependency. It shows the dependency in the AT direction. It is assumed that a recording scan frequency f.sub.W is 0.5 Hz, a reproducing scan frequency f.sub.R is 2.5 Hz and a skew is .+-.100 .mu.m. Assuming that the vibration in the AT direction is generated merely by the skew, the frequency dependency of the amplitude of the vibration in the AT direction in the record mode is represented by c. The amplitude is flat at 100 .mu.m at a frequency up to the recording scan frequency 0.5 Hz, and it decreases at a rate of -12 dB/oct at a frequency above the scan frequency. On the other hand, the frequency dependency of the amplitude in the AT direction in the reproduce mode is represented by d. The amplitude is flat at 100 .mu.m at a frequency up to the reproducing scan frequency 2.5 Hz, and it decreases at a rate of -12 dB/oct at a frequency above the scan frequency.
In order to keep the deviation of AT within .+-.0.1 .mu.m in both the record mode and the reproduce mode, the open loop gain G.sub.T of the AT servo is set as shown by .alpha. in FIG. 4. Namely, at a frequency below the reproducing scan frequency 2.5 Hz, EQU G.sub.T =20 log (100/0.1)=60 dB
and it decreases at a rate of -12 dB/oct at a frequency above the scan frequency.
As a result, sufficient AT control is effected in both the record mode and the reproduce mode. However, in the record mode, since the AT gain in a high frequency band is higher than required, the AT servo system is sensitive to a fine defect or dust on the surface of the optical card. This causes degradation of a recorded signal.
The same applies to the AF direction.